The variety of processes at the origin of life

Anyone who follows these pages knows that we have convincing data and models available to explain the evolution of the enormous past and present variety of life on our planet: starting from the heuristics and examples provided for the first time by Darwin in 1859 , the evolutionary sciences have contributed to the construction of a grandiose picture, to which both new pieces of detail and new large pieces of knowledge on the general mechanisms that are at work are added every day. For a long time, however, it has been difficult to identify a way in which life itself, and therefore the Darwinian process of evolution, could have been triggered. Darwin, who was already well aware of the problem and who regarded the matter as formidable difficulty, wrote very little about it. His vague hypothesis is famous, contained in a letter he wrote on 1 February 1871 to his longtime friend, the botanist Joseph Dalton Hookerin which he stated as the existence of a possible route for the birth of life “if (and oh how great if) we could conceive in some little warm pond with all sorts of ammonia and phosphoric salts, light, heat, electricity etc., that a protein compound has been formed chemically, ready to undergo even more complex changes.”

Only eighty years after this letter, the classic experiment by Miller and Urey, consisting in exposing a mixture of water, ammonia and methane to electric discharges, demonstrated forever how from simple and ubiquitous compounds, thanks to an energy source, it was possible obtain a complex mixture of amino acids, the components of proteins. Have we then come to understand how life originated on our planet? Was Darwin right? It is probable at this moment that we will never know, but not because there are no possible solutions to the problem: on the contrary, we have at this moment a variety of possible ways, all experimentally proven and all interesting, for, or at least of the first Darwinian replicator. What in particular, at the moment, seems difficult to know is precisely what came first: there are three main families of hypotheses.

The first identifies the starting point in the development of molecules capable of replicating by mistake, molecules that could have consisted of RNA or even of other chemical types. The second family of hypotheses instead focuses on the initial generation of vesicles capable of replication, starting from fats spontaneously formed in aqueous solution and in the presence of at least mechanical energy (capable of causing the division of the vesicles beyond a certain critical threshold). The third hypothesis is that of the initial start of complex cycles of chemical reactions in solution, cycles which would then have given rise to a sort of primordial metabolism, subsequently able to build the first cell and confine itself at least partially within it.

Now, it is interesting to note how all these processes have been reproduced experimentally under conditions that are plausible at least for some corner of the early Earth; but since we do not have access to the part of history on this planet that would allow us to really verify if and how widespread were the conditions necessary for each of the plausible chemical processes grouped in the three families of hypotheses just illustrated, the best we can do is demonstrate the processes that produce life and demonstrate that they are compatible with what we know about the early Earth, knowing that the degree of uncertainty about them is and will likely remain high. Moreover, many of the proposed mechanisms could have worked concurrently and simultaneously; the first steps could have taken place many times, in different ways and in different places on the planet, to then perhaps lead to the coalescence of some of these steps into something more complex. So what is the progress we have made in the 170-odd years since Darwin wrote the letter quoted at the beginning?

The point is precisely that we have identified a multitude of possibilities, all plausible for remote times and environments, and all with an important amount of experimental verification. We have also found that many of the hypothesized conditions for triggering the processes capable of generating at least the biochemistry of life are widespread in the universe, so much of the complex chemical material useful for the construction of life is scattered in the four corners of the cosmos. Thanks to this knowledge, we are also able to identify which are the extraterrestrial places closest to us and most promising for the triggering of alternative life to the terrestrial one: Europa, a moon of Jupiter, and Enceladus, one of Saturn, are icy bodies, with a large salty ocean below the surface and sources of energy and heat. Mars, at least in the past, has had even more promising conditions, and it is not impossible that we will find traces of at least a primordial biochemistry.

What has changed, therefore, is our general perspective: we do not know the precise details of the single process or of the set of competing processes which, in the insufficiently known conditions of the primitive Earth, gave rise to the first ancestor of all living things, but we know and have experimentally verified that the possibilities of triggering life, and with them also the probabilities, are much more than those of a single, fortuitous single process. There are many roads that can lead to life, and the universe is of boundless vastness; perhaps we will never find any form of extraterrestrial life close enough to us in spacetime to be identified, but there is no longer that lack of concrete possibilities why it may not have flourished elsewhere as well, and not just on Earth.